The following abbreviations are herewith defined:
3GPP third generation partnership project
CSG closed subscriber group
CSI channel state Information
DL downlink
eNB evolved nodeB (of an LTE system)
eICIC enhanced inter-cell interference coordination
E-UTRAN evolved UTRAN (LTE or 3.9G)
HARQ hybrid automatic repeat request
HeNB home eNB (base station)
LTE long term evolution of 3GPP
LTE-A long term evolution-Advanced
Node B base station or similar network access node
PDCCH physical downlink control channel
RRC radio resource control
TDM time-domain multiplexing (or time division multiple access)
UE user equipment (e.g., mobile equipment/station)
UL uplink
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
As the radio spectrum becomes more thoroughly utilized, geographic overlap among different radio networks becomes more prevalent. By example, in the LTE system (and LTE-A) there is the conventional or macro cell whose coverage area overlaps in whole or in part with that of a home network which serves a closed subscriber group CSG. One example of such overlap is shown at FIG. 1: the macro base station is the eNB 12 and the home base station is the HeNB 13. Three mobile devices are shown, in which UE 10-1 and 10-3 are under control of the eNB 12 and UE 10-2 is under control of the HeNB 13. Whereas the dashed line coverage area of the HeNB 13 is shown as being fully enveloped within the coverage area of the eNB 12 (the entirety of FIG. 1), it will be recognized that some deployments may exhibit only a partial overlap. As used further herein, the term eNB refers to the macro access node and the home access node will be specified as such to distinguish the two.
In the particular arrangement of HeNBs in the LTE system as well as similarly overlapping cells in other radio technologies, radio channels may be shared which gives rise to co-channel interference among the various UL and DL signals from the different but closely located radios. In LTE and LTE-A there is time-domain (TDM) enhanced inter-cell interference coordination (eICIC) which is applied between the eNBs and the HeNBs to reduce the co-channel interference between cells. For such cases it is also beneficial to optimize the channel state information (CSI) which the various UEs report on the UL to their respective access nodes, which enables the aforementioned TDM eICIC to also be optimized.
The TDM eICIC concept for LTE (and LTE-A) rests on the proposition that the HeNB 13 is only allowed to transmit in a sub-set of all DL subframes FIG. 2 is a table of DL subframes for the eNB and the HeNB which gives an example of the principle. The eNB is not restricted in which DL subframes it may transmit which is indicated by all DL subframes being shaded in FIG. 2 at the macro layer. The HeNB is restricted and is allowed to transmit only in the subset of DL subframes shaded in FIG. 2 for the HeNB layer, specifically subframes 1 through 4. At FIG. 2 subframes 5 through 8 are unshaded for the HeNB layer meaning they are almost blank. In this context, “almost blank” refers to cases with nearly no transmission from the HeNB and its transmissions are highly restricted (e.g., multi-media broadcast over a single frequency MBSFN is allowed in those DL subframes). In concept, the macro-UEs (under control of the eNB, perhaps those not allowed to connect CSG HeNB) which are close to the HeNB shall be scheduled during the time-periods with almost blank sub-frames from the HeNBs. By example, this means the eNB 12 should schedule UE 10-1 in any of subframes 5 through 8, which avoids that UE's DL signal from being exposed to too high interference. Other macro-UEs such as UE 10-3 could also be scheduled by the eNB 12 in other sub-frames.
For the TDM eICIC to operate properly, it is in generally assumed that the eNBs know in which sub-frames the HeNBs are muted. There has also been proposals in 3GPP discussions that the eNB signal to its own UEs which sub-frames are almost blanked (and therefore in possible use by the HeNBs).
The eICIC concept gives rise to several unresolved problems. First, for macro-UEs which are operating close to a non-allowed CSG HeNB such as UE 10-1 of FIG. 1, the CSI this UE reports on the UL to its eNB 12 will be significantly different depending on whether the reported CSI is measured during time-periods with almost blank sub-frames from HeNBs, or in other sub-frames. Second, in general the eNB 12 does not know the exact location of the UEs under its control, and so to adopt the scheduling for UE 10-1 noted by example above the eNB 12 must estimate its geographic location in order to determine whether or not it is close to some CSG HeNB 13 which it is not allowed to join.
There have been a few proposals to the 3GPP concerning CSI for TDM eICIC. Document R1-102353 entitled “Measurements and feedback extensions for improved operations in HetNets”, by Qualcomm (3GPP TSG-RAN WG1 #60 bis; 12-16 Apr. 2010; Beijing, China) proposes that the UE performs measurement on a set of subframes which the network signals, and that channel feedback is restricted to a single subframe. Limiting the feedback measurements to some specific subframes (e.g. either normal or almost blank) is intended to provide better feedback accuracy corresponding more directly to the TDM eICIC scheme in use. This appears to follow the CSI regimen for LTE Rel-8/9 which is specified in 3GPP TS36.213 v9.2.0 (2010 June). Specifically, the CSI reference resource is always a single subframe and the CSI is reported in an UL subframe spaced a fixed distance from the subframe which was measured, following the general HARQ timing (i.e. CSI measured in subframe n is transmitted in the UL subframe n+4).
Document R1-101981 entitled “Enhanced ICIC and Resource-Specific CQI Measurement”, by Huawei (3GPP TSG-RAN WG1 #60 bis; 12-16 Apr. 2010; Beijing, China) discusses a time/resource-specific CSI measurement, which limits the CQI averaging to some specific subframes depending on the network deployment model (e.g. HetNet). In practice this averaging appears quite problematic for the UE as it increases battery consumption and complicates memory handling since the measurements need to be buffered for multiple subframes.